Various topologies of power converting circuits are known in the art. Generally, in a power converting circuit, the input of the circuit is coupled to a power source (such as a battery or 3-phase AC source) and the output of the circuit is coupled to an electrical load to which the power provided by the power converting circuit is to be delivered.
Three phase AC-DC power converters are used to supply large amounts of power, while maintaining balanced operation of the three AC phase mains. In many of these three-phase circuits, one or more active semiconductor switching devices ("switches"), such as MOSFETS, JFETS, IGBTS, and thyristors, are coupled between the input and output of the power converting circuit. These switching devices are configured and selectively controlled (in a manner well known in the art) to switch on and off in such a way as to condition the power received from the AC power source for delivery to the electrical load--i.e., to provide power factor correction (PFC).
Several topologies and control methods for AC-DC power conversion have been developed during the past years. At least two types of topologies are available that provide low-cost, high-efficiency power conversion with simple control. The first topology may be referred to as the Single Switch Boost-type three phase PFC ("SSB converter"). A typical SSB converter uses a single switch, that is switched in zero current switching (ZCS) condition, to provide power factor correction. Such a topology is discussed in the publication entitled "An Active Power Factor Correction Technique For Three-Phase Diode Rectifiers" authored by A. R. Prasad, P. D. Ziogas, and S. Manias, IEEE PESC 1989, pp. 58-66.
The second topology is the Resonant Boost Input three-phase PFC ("RBI converter"). This topology is discussed in the publication entitled "Resonant Boost Input Three Phase Power Factor Corrector" authored by Da Feng Weng and Subbaraya Yuvarajan, IEEE APEC 1998 Record, pp. 958-62, and is hereby incorporated by reference in its entirety. A schematic diagram of this topology is shown in FIG. 1. The RBI converter works in zero voltage switching (ZVS) soft switching condition which allows the converter to work at higher switching frequencies. In this circuit, the equivalent duty-cycle of the boost inductor automatically varies with the input phase voltage. Thus, for a given value of total harmonic distortion (THD), the output DC bus 20 voltage can be relatively low and very close to the peak line voltage.
In the circuit of FIG. 1, components L.sub.a, L.sub.b, L.sub.c, C.sub.a, C.sub.b, C.sub.c and D.sub.1 -D.sub.6 form a three-phase input resonant network. The switches M.sub.1 and M.sub.2, along with diodes D.sub.a and D.sub.b, and inductor L.sub.1 are configured such that these components comprise a high frequency current source 10. This high frequency current source 10 operates to transfer energy stored in the input capacitors C.sub.a, C.sub.b, and C.sub.c (corresponding to each of the AC voltage input phases) to the output capacitor C.sub.dc (across which the desired load is connected). The capacity of the circuit to transfer energy will have an effect on the input power of the converter. In other words, this circuit may be used to control the input power and shape the waveform of the input current. This input waveform shaping function is described immediately below.
The input current waveform (i.e., the input current's magnitude, power and total harmonic distortion) provided by each of the AC voltage source input phases V.sub.a, V.sub.b, and V.sub.c is controlled by the variable excitation time for the corresponding input inductors L.sub.a, L.sub.b, and L.sub.c. For example, the input current waveform for current I.sub.a is controlled by changing the excitation time for input inductor L.sub.a. The excitation time for a particular input inductor is, in turn, controlled by changing the difference in the initial and final voltages on the corresponding input capacitors C.sub.a, C.sub.b, and C.sub.c during each switching cycle. (Note that the final voltage across an input capacitor during each switching cycle will be the final DC output voltage V.sub.dc and the initial voltage across the input capacitor may be as low as zero.) Clearly, the longer the time taken by the input capacitor to change from the initial to the final voltage during a switching cycle, the greater the excitation time for the input inductors. The time required by the input capacitors for charging from the initial to the final voltage may, in turn, be controlled by the input line current. Thus, it is clear that when the instantaneous amplitude of the input current is low, the time required for charging from the initial to the final voltage across the input capacitors will be relatively long, and when the instantaneous amplitude of the input current is high, the time for charging from the initial to the final voltage will be relatively short.
During each switching period, the energy supplied to the input capacitors C.sub.a, C.sub.b, and C.sub.c by the input currents must be released or transferred to the output of the converter, otherwise the capacitors C.sub.a, C.sub.b, C.sub.c cannot absorb any of the input power. In the converter, the high frequency current source 10 is used to transfer the energy stored in C.sub.a, C.sub.b, C.sub.c to the output of the converter.
The process of energy transfer by the high frequency current source can be divided into two stages. During the first stage, the energy in C.sub.a, C.sub.b and C.sub.c is transferred to the high frequency inductor L.sub.1. During the second stage, the energy in L.sub.1 is transferred to the output of the converter through the active switches M.sub.1 and M.sub.2. Because the energy stored in C.sub.a, C.sub.b, C.sub.c is transferred to the output of the converter through L.sub.1, the current in L.sub.1 must be relatively high to store the total energy of C.sub.a, C.sub.b and C.sub.c. As shown in FIG. 1, the current in L.sub.1 also passes through active switches M.sub.1 and M.sub.2. The amplitude of the current will affect the conduction loss of the active switches M.sub.1 and M.sub.2. In the second stage, as the active switches M.sub.1 and M.sub.2 alternately turn on or off, the energy in L.sub.1 will be released to the output through the body diode (not shown) of M.sub.1 or M.sub.2 and diode D.sub.a or D.sub.b. The turn-off current of the active switches M.sub.1 and M.sub.2 will be the maximum value of the current in L.sub.1. The amplitude of the current will also affect the switching loss of M.sub.1 and M.sub.2.
During each switching period, the discharge time of C.sub.a, C.sub.b, and Cc should be less than half of the switching period in order to guarantee that C.sub.a, C.sub.b, and C.sub.c will be charged from zero again. For a given the value of (1) C.sub.a (=C.sub.b =C.sub.c), (2) the switching frequency (f.sub.s), (3) the rated input power, and (4) the output voltage V.sub.dc, the minimum amplitude I.sub.SM of the high-frequency current source required to transfer the energy can be expressed as: EQU I.sub.SM =8 f.sub.s C.sub.a V.sub.dc (1)
The relation between the product f.sub.s C.sub.a and the input power level is obtained as EQU f.sub.s C.sub.a =F/R.sub.equ (2)
where F is a coefficient and R.sub.equ is the equivalent resistance seen by the capacitor C.sub.a. From (2), it is clear that the product of f.sub.s C.sub.a varies linearly with the input RMS current. From equations (1) and (2), it can be concluded that, as the power level of the converter increases, the amplitude of the high-frequency current source 10 also has to be increased to guarantee the required input power and a sufficiently low THD in the three-phase input current.
The topology just described has several advantages over the previously mentioned SSB converter topology. This RBI converter topology, unlike the SSB converter topology, operates in zero voltage switching (ZVS) condition and thus can handle higher switching frequencies than the SSB converter. Additionally, much lower DC bus 20 voltages are possible for a given AC input voltage and given acceptable level of total harmonic distortion. However, this RBI converter topology has one disadvantage as compared to the SSB converter. In the RBI converter, the current stress on the switches is almost double that of the SSB converter. Increased switch current stress leads to undesirable conduction losses and turn-off switching losses. Thus, there is a need for an AC-DC power converter that has all the advantages of the RBI type converter while having a switch current stress approximately the same as an SSB converter.